Prep for 2.0 release candidate 1

This PR contains some preparatory fixes in preparation for `v2.0.0-rc1`. The most important thing here, perhaps, is documenting the release procedure ([read here](https://github.com/nengo/nengo/blob/03c75c592a9ea59ada5bb97338b4136536bac7d1/docs/releasing.rst)); the rest of this PR is doing Stage 1 from that documents.

1. #590 should be fixed up and merged before reviewing / merging this PR.
2. Once this PR is merged, I will do Stages 2 and 3 described in the how-to release document for `v2.0.0-rc1`.

Then we'll have a release candidate that we can test on and document!